The present invention is related to Japanese patent application No. Hei. 11-291846, filed Oct. 14, 1999; the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a catalyst deteriorating state detecting apparatus, and more particularly to a catalyst deteriorating state detecting apparatus for determining a deteriorating state of a catalyst based on a storage quantity or the magnitude of saturated absorption of the catalyst.
2. Description of the Related Art
To detect a deteriorating state of a catalyst used for cleaning exhaust gas or detect a decrease in storage quantity of the catalyst, a dither period and the amplitude at a target air-fuel ratio are gradually increased and a deteriorating state or the storage quantity of the catalyst is determined based on the dither period and the amplitude. In response, a change in output is generated by an air-fuel ratio sensor provided on the downstream side of the catalyst. Such a device is disclosed in JP-A No. Hei7-1030309. Another method is disclosed in JP-A No. Hei6-17640. Here, while repeatedly alternately changing a target air-fuel ratio to rich and lean, a correction magnitude and a correction period of the target air-fuel ratio are increased gradually and a deteriorating state of the catalyst is recognized from computation of a storage quantity in the catalyst based on a correction magnitude and a correction period. This results in a change in output generated by an air-fuel ratio sensor provided on the downstream side of the catalyst.
With conventional methods, however, the storage quantity of a catalyst cannot be computed with a high degree of precision. Even for the same variation pattern of the target air-fuel ratio in calculating the storage quantity of a catalyst, the time change in output generated by an air-fuel ratio sensor provided on the downstream side varies with change in intake-air flow rate or variation in exhaust-gas flow rate. Thus, by calculating a catalyst storage quantity based on a dither period and an amplitude or a correction magnitude and a correction period, computation of storage quantity varies depending on the flow rate of intake air or the flow rate of exhaust gas. This reduces precision in detection of a deteriorating state of the catalyst.
In addition, to improve the efficiency exhaust gas cleaning processes during engine operation, the air-fuel ratio of the exhaust gas is controlled to a value close to the stoichiometric air-fuel ratio. In calculating the storage quantity of a catalyst, however, the air-fuel ratio is repeatedly changed toward rich and lean to change the downstream side air-fuel ratio toward rich and lean. As a result, conventional methods result in poor exhaust gas emissions.
It is thus a first object of the present invention to improve the precision of determining a catalyst""s storage quantity.
It is yet another object of the present invention to allow the storage quantity of a catalyst to be computed or a deteriorating state of a catalyst to be detected without worsening emission of exhaust gas.
To accomplish these and other objects, a catalyst deteriorating state detecting apparatus is provided for calculating a catalyst storage quantity based on the flow rate of exhaust gas into the catalyst between a change in output generated by an upstream sensor and a change in output generated by a downstream sensor due to a variation in target air-fuel ratio. A deteriorating state determining means then determines whether a deteriorating state of the catalyst exists based on the storage quantity calculated by the storage amount computing means.
Here, the storage amount computing means calculates a storage quantity of a catalyst based on the flow rate of exhaust gas flowing into the catalyst during a change in output generated by the upstream sensor and a change in output generated by the downstream sensor, to eliminate the effect of a change in intake-air flow rate or a change in exhaust-gas flow rate during calculation. In another aspect of the invention, a storage quantity is calculated based on the flow rate of exhaust gas flowing into the catalyst during a change in output generated by an upstream sensor to a predetermined value and a change in output generated by a downstream sensor to another predetermined value. That is, during a change in output generated by the upstream sensor to a predetermined value and a change in output generated by the downstream sensor to another predetermined value, a rich-component quantity (or a lean-component quantity) flowing into the catalyst changes depending on the air-fuel ratio of exhaust gas besides the flow rate of intake air (or the flow rate of the exhaust gas). Thus, the storage quantity is calculated based on a change in exhaust-gas flow rate and a change in target air-fuel ratio.
In another aspect, a storage quantity is calculated by switching a target air-fuel ratio to a rich value right after the end of a fuel-cut state. That is, during a fuel-cut state, the air-fuel ratio of exhaust gas flowing into the catalyst is lean, causing the storage quantity of the catalyst to change to a lean value. Thus, a storage quantity is calculated by switching the air-fuel ratio of the exhaust gas to a rich value to improve the efficiency of exhaust gas cleaning. As a result, the storage quantity is calculated or a deteriorating state of the catalyst is detected without worsening emission of exhaust gas.
When the period of a fuel-cut state is a certain length, the amount of absorption of a lean component such as O2 in the catalyst during the fuel-cut state becomes saturated, and the magnitude of the saturated absorption becomes the storage quantity of the catalyst. Thus, with the absorption magnitude of the lean component due to a fuel-cut state placed in saturated state, if the storage quantity is calculated from a flow rate of exhaust gas flowing into the catalyst during target air-fuel ratio switching to a rich value right after the end of the fuel-cut state and a point of time the air-fuel ratio on the downstream side of the catalyst reaches a stoichiometric air-fuel ratio value, a storage quantity of the catalyst computed with a high degree of precision can be obtained.
In light of the above, another aspect of the invention calculates a storage quantity of the catalyst right after the end of a fuel-cut state when the absorption magnitude of a lean component in the catalyst during the fuel-cut state has attained or exceeded a predetermined value or a saturated level. In this way, the storage quantity of the catalyst can be computed with a high degree of precision. According to another aspect of the invention, a storage quantity of the catalyst is calculated by setting the target air-fuel ratio at a new value dependent on a computed value of the storage quantity obtained in the immediately preceding calculation of the storage quantity. Here, if the target air-fuel ratio is increased to a rich value when the storage quantity of the catalyst decreases, the catalyst could become saturated with a rich component in a relatively short period of time. This rich component just passes through the catalyst. Thus, by setting the target air-fuel ratio at a new value dependent on a computed value of the storage quantity obtained in the immediately preceding calculation, the target air-fuel ratio can be set at a value appropriate for the current storage quantity. As a result, even if the storage quantity decreases to a small value, the small value of the storage quantity can be calculated without worsening emission of exhaust gas.
In another aspect, the storage quantity is computed by correction of the target air-fuel ratio depending on the flow rate of intake air. At a fixed target air-fuel ratio, the larger the flow rate of intake air, the flow rate of exhaust gas, the greater the quantity of a rich component flowing into the catalyst and, hence, the shorter the time to catalyst saturation with the rich component. Thus, by correcting the target air-fuel ratio according to the flow rate of intake air, that is, by setting the target air-fuel ratio at a value appropriate for the flow rate of intake air, the value of the storage quantity can be calculated without lowering emission of exhaust gas even if the storage quantity decreases to a small value.
In another aspect, the storage quantity is calculated by correcting a feedback control gain of the air-fuel ratio depending on the flow rate of intake air. Here, the response of the feedback control of the air-fuel ratio, that is, the follow-up characteristic, can be changed according to the flow rate of intake air. In another aspect, the storage quantity is calculated at low load, such as idle. At low load, the exhaust gas flow rate decreases so that a temporary change in target air-fuel ratio does not have an adverse effect on the exhaust gas emission.
In another aspect, an abnormality other than a deteriorating state of the catalyst is detected based on a computed value of the storage quantity. The system described above can be applied systems having one or more catalysts on the upstream and downstream sides of the exhaust gas path. Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.